The marine heatwave bulletin provides forecasts and analysis of marine heatwave events across the globe and throughout the year. Used datasets include observations (satellite sea surface temperature maps) and numerical model analyses (assimilating satellite and in situ observations) to derive marine heatwave forecasts for a 10-day period. [1] This week’s forecasts were produced using as a comparison the marine heatwave situation on 04/03/2025.

Forecasts for 15 March

Indian Ocean

The marine heatwave present east of Madagascar remains stable in intensity with moderate and strong categories, but is decreasing in extent. The marine heatwave present in the Arabian Sea is intensifying as the surface area of the strong categories increases, and the local appearance of a severe category. The marine heatwave present in the Bay of Bengal remains generally stable. The marine heatwave present northwest of Australia is decreasing in intensity and is now in the moderate and strong categories.

Tropical Pacific Ocean

The marine heatwave in the western part of the basin, affecting Indonesia, the Philippines and Papua New Guinea, is decreasing in intensity and is now generally in the moderate category. The marine heatwave in the central part of the basin, from the Philippines to the North American coast, is decreasing in intensity to moderate overall, as well as in extent. The marine heatwave to the east of the basin, off the coast of Ecuador, remains stable overall, with moderate and strong categories.

South Pacific Ocean

The marine heatwave in the center of the basin between 180°W and 120°W is decreasing in extent and intensity, with fewer areas in the strong category. The marine heatwave off the Chilean coast is decreasing in extent, but remains moderate overall.

Tropical Atlantic Ocean

The marine heatwave in the Caribbean Sea is intensifying, with most of it in the strong category, and some in the severe category. The marine heatwave to the west of the Gulf of Guinea is intensifying, with an increase in the area covered by strong categories.

European Zone

Mediterranean Sea

The marine heatwave present in the center of the Mediterranean basin is intensifying and is now mostly categorized as strong, with locally severe categories. Moderate categories persist to the east and west of the basin.

Weekly Temperature Anomalies

09/03/2025-15/03/2025

Mean sea surface temperature anomalies in each global ocean region for the week 9 to 15 March, 2025.

Access the Daily Global Physical Bulletin for a 9-day forecast here.

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What are marine heatwaves?

Marine heatwaves (MHW) are extreme rises in ocean temperature for an extended period of time. They can occur at different locations in the ocean, and their magnitude and frequency have increased over the last couple of decades, with harmful impacts on ecosystems, and human activities. According to the latest report released by the Intergovernmental Panel on Climate Change (IPCC AR6 SYR), it is found with high confidence that in the near-term at 1.5°C global warming, the increasing frequency of marine heatwaves will increase risks of biodiversity loss in the oceans, including from mass mortality events.[2]

How are marine heatwaves calculated? 

A marine heatwave is an episode during which the ocean temperature is abnormally warm for at least 5 consecutive days.

Adapted from Hobday et al. (2018)

For any location in the ocean, the normal temperatures are defined for every day of the year using a climatological period (here 1993–2016). A heatwave is identified when the measured daily temperature is within the top 10% of the highest recorded values for that day  (i.e., above the 90th percentile, see diagram), and with this condition persisting for at least five consecutive days.

The intensity of the heatwave on any given day is measured as the number of degrees above the climatological average (represented by the bold black line) indicated by the blue arrow. We can either calculate the cumulative intensity over the entire heatwave or record the maximum intensity.

Heatwaves are classified based on their intensity level. To do this, the intensity is compared to the difference between the climatological value and the 90th percentile value. A mhw intensity between 1 and 2 times this difference corresponds to a heatwave of moderate category; between 2 and 3 times, to a strong category; between 3 and 4 times, to a severe category; and a difference greater than 4 times corresponds to an extreme category.

Notes

Datasets and products :

 Global Ocean Physics Analysis and Forecast (sea surface temperature) E.U. Copernicus Marine Service/ Mercator Ocean International. https://doi.org/10.48670/moi-00016

[2] IPCC AR6 SYR chapter 4.3 https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf